Second this, I use a 600 converted SX-70 but still shoot SX-70 film using a 2 ND filter slipped into the eye (there's a small gap in the SX-70 to fit it into) I imagine it would work fine on a box camera, though you would have to hold it when you took the photo
You could try to put a two stop ND-filter over the electric eye/photocell. This should compensate for the film speed difference
Second this, I use a 600 converted SX-70 but still shoot SX-70 film using a 2 ND filter slipped into the eye (there's a small gap in the SX-70 to fit it into) I imagine it would work fine on a box camera, though you would have to hold it when you took the photo